1. Field of the Invention
The present invention relates in general to a method of and an apparatus for searching for a fiducial mark, and more particularly to a fiducial-mark search on the basis of an image.
2. Discussion of Related Art
For example, fiducial-marks are provided on a circuit substrate (e.g., a printed-wiring board) on which electric components (typically, electronic components) are to be mounted. The fiducial marks are used to detect errors of positioning (hold-position errors) of the circuit substrate as held by a circuit-substrate holding device. In an electric-component mounting system arranged to mount the electric components on the circuit substrate, an imaging device is usually provided to take images of the fiducial marks provided on the circuit substrate, and the positions of the fiducial marks are detected on the basis of image data indicative of the images of the fiducial marks. The thus detected positions of the fiducial marks are compared with nominal positions of the fiducial marks, to detect the hold-position errors of the circuit substrate. When the images of the fiducial marks are taken, areas surrounding or near the fiducial marks are also imaged, so that the images taken must be searched to find the images of the fiducial marks. Namely, the images of the fiducial marks must be distinguished from the images of the surrounding areas. To this end, search patterns are conventionally used.
The fiducial marks used have different shapes or configurations, so that there are prepared a plurality of different reference patterns, one of which has the same shape as the desired mark-mark to be searched for is selected by the operator of the system. The operator then manipulates an input device to supply a control device with data indicative of the dimensions of predetermined portions of the selected reference pattern. When a search for the fiducial-mark in question is made, a search pattern is generated on the basis of the selected reference pattern and its dimensional data, so that the search is made using the generated search pattern. One of the images taken by the imaging device, which matches most the search pattern, is determined as the fiducial mark in question.
Where the number of mark marks having respective different shapes is comparatively small, appropriate search patterns can be easily generated using the reference patterns. Where the number of fiducial marks having different shapes is comparatively large, it is difficult to prepare the reference patterns for all of the fiducial marks having the different shapes. If the reference patterns for some of the fiducial marks are not available, the search patterns for these fiducial marks cannot be generated.